When I tell patients that caffeine can stall weight loss, they sometimes look at me funny. After all, coffee has become a health food in many circles. I know—it borders on sacrilegious to some. Why would I speak so badly about this beloved beverage?

Yet some of these patients find that dramatically reducing caffeine intake (and becoming more mindful about its impact) can be a game-changer to reach their goal weight as part of a whole-body detox program. I sometimes use coffee and caffeine interchangeably because coffee accounts for most of the caffeine intake among most Western people. (Tea is more prevalent among Eastern countries.) Let's face it, next to a glass of wine, Americans love their coffee.

And thanks to its high amounts of polyphenols and other antioxidants, coffee indeed enjoys a health glow these days. In fact, coffee is the No. 1 source of antioxidants for many people, who neglect to get them from other important sources, like vegetables. Research shows drinking coffee—sometimes, lots of coffee—can lower risk for certain conditions, including type 2 diabetes and Parkinson's disease.

Considering this and other evidence, you might wonder why I'm arguing caffeine can make you gain weight. I promise I have nothing against coffee!

The dark side of coffee.

Yet caffeine and coffee have a dark side (no pun intended). The average person gets about 300 milligrams (mg) of caffeine daily from coffee, tea, soft drinks, chocolate, and energy drinks. This is about the amount of caffeine in a venti Americano. These therapeutic, drug-dose amounts can tally up quicker than you might imagine. A venti Starbucks Blonde Roast is even stronger, packing a whopping 475 mg of caffeine. If you're tempted by the chain's free refills, you can easily get a gram or more of caffeine in one sitting.

Elevated blood sugar levels also spike insulin levels. Insulin is an anabolic or storage hormone. One thing this hormone is very good at storing is fat in your midsection. Keeping insulin elevated can make your cells less sensitive to its "signal"; this is called insulin resistance. To lose weight and keep it off, you want your cells to be the complete opposite—insulin sensitive.

Therefore, coffee (caffeine) has the potential of making your cells more insulin resistant. One systematic review and meta-analysis looked at seven qualifying studies and concluded that caffeine reduces insulin sensitivity in healthy subjects in the short term, potentially creating high blood sugar.

These results often happen in a dose-dependent matter. What this means is drinking seven cups of coffee will have a more dramatic impact on blood sugar and insulin levels than drinking two cups. But for some people, it doesn't take much: One study among healthy men and women found caffeine disrupted insulin sensitivity in a dose-dependent manner with no male or female preference but began at very low doses.

In all fairness, while caffeine can impair insulin sensitivity, researchers surmise the polyphenols in tea, coffee, and other beverages might balance out those effects. Maybe. Truthfully, studies about caffeine's impact on blood sugar, insulin, and health factors like obesity and diabetes are all over the map.

Why the inconsistencies? For one, some of the research has methodological flaws. Some studies didn't weigh other variables, for instance, that might also affect blood sugar and insulin levels.

Two, looking at the short-term effects of how caffeine affects glucose and insulin levels can't always explain the long-term effects of how caffeine (and coffee in particular) can affect your risk for chronic disease including diabetes. However, short-term disruptions in insulin sensitivity and blood glucose levels, which lead to weight gain in the form of increased abdominal fat, then deposits a feed-forward mechanism in your abdomen that propels the forces of weight gain, even as blood sugar levels may have normalized with long-term caffeine consumption. In other words, one problem creates another problem that continues to fuel the fires of inflammation and weight gain.

And people metabolize caffeine differently, which can skew study results. "Why is there so much conflicting evidence about coffee? The answer may be in our genes," writes Anahad O’Connor in the New York Times. He notes that slow metabolizers (caffeine hangs out in their system longer; I'm one of them ) have a higher likelihood of creating health problems. Fast metabolizers, on the other hand, clear caffeine more quickly, meaning they absorb the antioxidants and other good stuff in coffee without the potential problems of caffeine. In short, people like me best stay away from the most potent forms of caffeine—like coffee.

What's a coffee-loving person supposed to do?

I'm not arguing you should forever give up your beloved morning cup of coffee or mid-afternoon tea, which for some patients literally becomes the best part of waking up or a break in their otherwise-hectic day. For myself, I enjoy a cup of tea with its balanced antioxidant polyphenols and L-theanine for calming the nerves.

At the same time, I want you to be aware that too much caffeine—and for some people, it doesn't take much—can raise blood sugar and insulin levels, which will result in weight gain. That's why cutting out caffeine in the form of coffee is part of any detox program, including my Happy Gut 28-Day Cleanse. In the meantime, let me help you enjoy your cup of joe while balancing its health risks and benefits.

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